A system of this kind has been disclosed for example in the periodical "Sensor Review" October 1982, pages 180 to 184. It is based on a triangulation method and on the tracking of the head of a robot arm by two separate laser beams. By measuring the compound angles of the two beams and the tracking error, the distance separating the two laser sources being known, the robot arm position can be calculated. Tracking is achieved by transmitting the beam back onto the light sensitive detector which, in this case, is a four-quadrant detector. The electrical signals derived from the detector of each sub-system are used to correct the direction of the associated laser beam such that the beam tends to point directly to the reflecting mirror attached to the head of the robot arm.
The performance of such a system strongly depends on the precision and the speed of the mirror positioning. If this speed is low, the beam strikes the target so far away from its center that the reflected beam is no longer received by the light sensitive detector. The precision of measuring the beam angle limits the resolution that may be achieved in a measuring volume. The resolution diminishes as the volume increases in size.
The system disclosed in the above cited periodical employs moving-iron galvanometers for pivoting and the present mirror position is reported by capacitive position transducers. Experiments have shown that such pivoting means suffer from several drawbacks and thus considerably restrict the measurement accuracy, the repeatability, the tracking speed, the tracking stability and the operational volume.
This is probably due to the following effects:
(a) The mirror is imperfectly mounted between ball bearing supports which thus induce a so-called wobble error, PA0 (b) there has been observed a hysteresis between the mirror movement and the electrical control signals for controlling this movement, PA0 (c) the capacitive transducers do not procure the high precision required.
It is the object of the invention to improve the known target tracking system such that the measurement accuracy, the speed of the target and the working volume of the target can be increased without affecting the tracking stability.